1. Field of the Invention
This invention relates to a process for recovering oil from tar sands. More particularly, the invention relates to a process whereby tar sands are extracted with water to produce a bitumen-rich layer and a bitumen-lean layer containing relatively more water and solids than the bitumen-rich layer and the bitumen-lean layer containing solids including clay having adhered organic matter is sent to a pyrolysis zone to provide for increased oil recovery.
2. Description of the Prior Art
Among the many approaches considered for separating the hydrocarbon fraction from tar sands, the aqueous extraction process represents a well-developed recovery technique. Typically, the tar sands are contacted with hot or cold water to form (i) a bitumen-rich layer containing bitumen, water and solids including sand and clay having adhered organic matter, (ii) a bitumen-lean layer containing relatively less bitumen and more water and solids than the bitumen rich layer and (iii) precipitated, relatively bitumen free sands. The water and solids are separated from the bitumen-lean layer and the resulting bitumen-lean stream is combined with the bitumen-rich layer which is thereafter diluted with naphtha, allowed to settle and then centrifuged to remove water and residual solids. After removal of the diluent, the bitumen is fed to a pyrolysis unit wherein the bitumen is heated to form distilled and cracked products including vaporized liquid oil products, normally gaseous products and carbon which is deposited on solids present in the pyrolysis zone.
One of the principle disadvantages of the tar sands hot and cold water extraction processes is the enormous volume of aqueous tailings. These tailings contain a stable suspension of inorganic fines. Since no economically viable schemes have been devised for removing these suspended fines, the tailings are held in sludge ponds which are both a major expense and potentially an environmental hazard.
A further disadvantage of the aqueous extraction process is the loss of oil present as adhered organic matter in the finely divided clay which is in admixture with the separated sands. The presence of organics in these clays is reported in Energy and Fuels 1988(3) 386-391.
Various solvent extraction schemes have been proposed as alternatives to the aqueous extraction of tar sands. For example, Hanson discloses in U.S. Pat. No. 4,071,433 a liquid slurry process for extracting tar sands in which the tar sands are slurried with an oil and divided in a centrifuge into streams containing course and fine sands. The fine sands stream is fed to a coker where the fines act as a nuclei in coke formation. The course sands stream is filtered by means of a hot oil filter and subsequently dried. Similarly, Irani et al. disclose in U.S. Pat. No. 4,036,732 the use of a C.sub.5 -C.sub.9 paraffin hydrocarbon solvent for the countercurrent extraction of tar sands.
Other references describe a non-extraction method for removing oil by the direct distillation of oil from bituminous sand in a fluidized solids bed. For example, Peterson and Gishler describe in The Petroleum Engineer, April, 1951, at pages 66-74 a fluidized solids technique for recovering oil from Alberta bituminous sand. In this process, raw bituminous sand is fed into a fluidized solids bed to distill and crack the bitumen present in the bituminous sand.
A review of the various known processes for recovering oil from tar sands is given by Chrones and Germain in their article entitled Bitumen and Heavy Oil Upgrading in Canada, Fuel Science and Technology International, 7(5-6), 783-821(1989).